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EducationWhat happened to this copper cube?

2nd Dec 2018 17:14 UTCLarry Maltby Expert

This is a Tom Rosemeyer specimen from the South Kearsarge Mine that I photographed some years ago (FOV 4.5 mm). It appears that the crystal lattice did not fill on the cube edges. Is there a crystallographic reason why the lattice did not fill on the diagonal?

3rd Dec 2018 01:26 UTCLarry Maltby Expert

Another strange copper crystal that appears to be cracked. This one was found in a rock on the beach near Copper Harbor when etched in acid (FOV 4.0 mm). Any ideas how this formed?

3rd Dec 2018 12:12 UTCReiner Mielke Expert

The edges are more prone to dissolution and that looks like what happened. Cracking is sometimes due to alteration which can change the volume of the mineral being altered and cause cracking.

3rd Dec 2018 14:48 UTCTom Rosemeyer

Reiner:

Thank you for you comment on my specimen of the copper cube that Larry Maltby posted. I'm working on a full length article on "copper wires" from the Michigan Copper County that will be in Rocks & Minerals magazine. Do you have any thoughts on why there was an accelerated growth in one direction to form wires rather than crystals.

3rd Dec 2018 15:18 UTCLarry Maltby Expert

Here are a couple of copper wire photos of a Rosemeyer specimen from the Allouez Mine to support his inquiry.

3rd Dec 2018 18:41 UTCSusan Robinson

Larry,

Beautiful photos of Tom's specimens, as usual. One possibility might be related to a general axiom of crystal growth that the largest faces present on a crystal are usually the slowest to grow. This is easily explained in cubic crystals, especially in copper, since only one kind of atom is involved. As the crystal begins to grow, it needs to attract copper atoms from 3 different directions: from top or bottom, right to left, or front to back. The large, flat plane of the cube face effectively can attract from only 1 direction (perpendicular to it). Along the cube edge, which is like a stair step, it can attract from 2 different directions, and at the corners of the cube, it can attract from all 3 directions. Therefore, the corners deplete the solution of copper atoms the fastest, followed by the edges, and finally, the large cube face, which is taking the longest time to grow, ends up being the largest form present.

The trace of the diagonal line running along the lower right cube face approximates the trace of an old octahedron. In the ideal world, this shouldn't appear. However, one explanation, and I have no idea if this is right or not, might be related to a diminishing supply of copper atoms in the solution in which the crystal is growing. As the crystal uses up the available copper, it can no longer develop, leaving the growing edges clearly present, rather than forming a perfect cube with no other forms showing.

George Robinson

3rd Dec 2018 19:19 UTCPaul Brandes 🌟 Manager

I was hoping you would post something to this thread at some point, George. Do you believe what you posted above applies to the elongated copper cubes that were found several years ago on the Peninsula as well? Was there ever a consensus on how those came to be?

In looking for some answers, I ran across a recently published (2018) report on the growth of native silver wires in nature. I'm wondering if some of that research could also be applied to copper wires as well, given that they are both native elements and tend to form in approximately the same environments? Below is the link to that report.

Microstructural Insights into Natural Silver Wires

3rd Dec 2018 20:52 UTCReiner Mielke Expert

I see no reason why the growth mechanism for copper should be different than silver.

3rd Dec 2018 21:02 UTCAlfredo Petrov Manager

I don't see a reason either, but there must be some, otherwise why for example would silver wires be so common and copper wires so rare?

3rd Dec 2018 21:30 UTCLarry Maltby Expert

Thanks George and Susan for the detailed information. This would also go a long way to explain “hoppered” copper crystals. I never related the diagonal line to the octahedron but the angle sure looks right. I have seen quite a few “hoppered” specimens, some of them spectacular, but this crystal is unique. Leave it up to Tom to come up with these puzzlers. Here is another photo from the same specimen that shows a thick coating of copper with epidote terminations poking through.

4th Dec 2018 03:03 UTCDoug Daniels

If we're considering copper and silver, why not throw gold into the mix? It's in the same group on the periodic table (1B), and also forms wires (as well as actual crystals, and "leaves"). Maybe something to do with the atomic structure of each metal and associated electrical forces, as the atomic size kinda increases as you go from copper to silver to gold. Just grasping at straws (do any of them form straws?).

4th Dec 2018 11:19 UTCDavid Von Bargen Manager

You can also add in the very rare native lead wires.

Silver and copper wires are probably generated by different processes. The silver ionic conduction in acanthite is very high. It only takes a little added energy to get silver wires to grow on acanthite (growing from the surface of the acanthite crystal). Silver will also form extremely elongated crystals (I have a specimen from Jáchymov that consists of dendritic texture that is composed of densely packed, elongated crystals).

https://gsa.confex.com/gsa/2018AM/webprogram/Paper320877.html

4th Dec 2018 11:44 UTCLarry Maltby Expert

This copper wire was photographed at Wayne State University in 1974. It is an old specimen that seems to show elongated crystal surfaces. The black crystal was not identified but it looks like a tarnished silver crystal.

This specimen was not part of the Goddard Collection so it is not so well documented. My notes give the location as the “Keweenaw”. The field of view is 2.5 cm.

4th Dec 2018 16:31 UTCDonald B Peck Expert

Could the tendency to form wires be related to the ionic charge to diameter ratio? I have no idea how or why, but it is significantly different.

6th Dec 2018 14:49 UTCLarry Maltby Expert

Here is another strange silver crystal from the Keweenaw. It appears to be an elongated cube that abruptly changed to an elongated octahedron creating a “silver scepter?”. It is from the Mass Mine (FOV 3.2 mm).

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